target/riscv: Introduce DisasExtend and new helpers

target/riscv: Introduce DisasExtend and new helpers

Introduce get_gpr, dest_gpr, temp_new -- new helpers that do not force
tcg globals into temps, returning a constant 0 for $zero as source and
a new temp for $zero as destination.

Introduce ctx->w for simplifying word operations, such as addw.

Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210823195529.560295-6-richard.henderson@linaro.org
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>